Date: Fri Oct 28 15:45:59 2005
From: andrey at securityelf.org (Andrey Bayora)
Subject: Re: Multiple Vendor Anti-Virus Software
	DetectionEvasion Vulnerability through forged magic byte

Hello x,

> The AV vendors aren't going to patch their products if they
> don't detect your PoC; they're just going to write a new
> signature or modify an existing signature to detect your
> new variants.  The fact that it can and will be fixed by
> AV signatures instead of product patches should help you
> figure out if this is a product vulnerability issue or just
> a "new virus variant" issue.

Good point, so I have news for you - some AV vendors contacted me and they
are WILL issue patches for their products. Is it what you need as a proof of
existence of a bug? Please, wait couple of weeks.

> BTW, Andrey, did you bother to use the "deep scan",
> "heuristic mode", "reviewer mode", etc to see if any
> of those AV scanners picked up your new variants?

YES, that is the reason why I prefer to use my AV lab instead of
virustotal.com and others. The only exception is CA - I tested 7.0 version
that didn't has "reviewer mode" (or I didn't found how to enable this).

> I bet you didn't.

Why are you guessing (betting)? I provide all information that you need to
check this bug and not to make up a conclusions based on guesses.

Best regards,
Andrey Bayora.



----- Original Message ----- 
From: "x" <x@...ckopssec.com>
To: <full-disclosure@...ts.grok.org.uk>
Sent: Friday, October 28, 2005 8:05 AM
Subject: [Full-disclosure] Re: Multiple Vendor Anti-Virus Software
DetectionEvasion Vulnerability through forged magic byte


>
> Andrey Bayora said:
> + > If your altered virus sample
> + > still executes correctly, you have simply created a new
> + > virus variant.
> +
> + Not exactly, please look at this virustotal.com log
> + http://www.securityelf.org/updmagic.html
> +
> + The altered (120 bytes prepended) TXT_* variant is STILL
> + detected by your product (CA), but when I change the first
> + byte from "Z" to "M" - your product fails (MZ_* variant).
> + I believe, that if I PREPEND 120 bytes to known virus and
> + the virus is still detected with the SAME signature -
> + then I DID NOT create a new variant. Now one more example:
> + try to change the first byte "Z" in the TXT_* variant to
> + any value, but not to "M" - this virus will be detected,
> + but when you change to "M", thus creating the .EXE magic
> + byte - the variant is not detected !!!  My conclusion:
> + the antivirus "thought" that the file is the executable
> + type instead of determining the file type by the
> + extension.
> +
> + That is my point, if you still think that your product is
> + OK - do not do anything.
>
> Thierry Zoller said:
> + WJK> You are effectively altering existing viruses to the
> + WJK> point that AV scanners do not detect them.
> +
> + No, he is changing a few bytes only.
> +
> + WJK> If your altered virus sample still executes
> + WJK> correctly, you have simply created a new virus
> + WJK> variant.
> +
> + No, there is no variant, the virus executes EXACTLY as
> + before. A variant acts differenlty then a precedent
> + version, else it would be no variant. To your AV engine it
> + is a variant, yes, but only because it is flawed.
>
> Why are you guys having such a difficult time comprehending
> this?  Read both the general and AV-specific definitions of
> the word "variant".
>
> http://dictionary.reference.com/search?q=variant
> http://www.symantec.com/avcenter/glossary/index.html#v
> http://us.mcafee.com/VirusInfo/VIL/glossary_app.asp#v
>
> If you take an existing virus and modify it, you have
> created a variant.
>
> The AV vendors aren't going to patch their products if they
> don't detect your PoC; they're just going to write a new
> signature or modify an existing signature to detect your
> new variants.  The fact that it can and will be fixed by
> AV signatures instead of product patches should help you
> figure out if this is a product vulnerability issue or just
> a "new virus variant" issue.
>
> BTW, Andrey, did you bother to use the "deep scan",
> "heuristic mode", "reviewer mode", etc to see if any
> of those AV scanners picked up your new variants?  I bet
> you didn't.
>
> Thierry Zoller said:
> + WJK> Consequently, the issue that you describe is *not* a
> + WJK> vulnerability issue, but rather just an example of a
> + WJK> new variant that has not yet been added to an AV
> + WJK> vendor's database of "known viruses".
> +
> + Thank you James, this _to my knowledge_ (perhaps the guy
> + from vmyths knows better) is the first time the complete
> + failure of todays AV solutions is shown naked publicaly
> + directly by a representant of an AV company. This
> + statement coming from a AV vendor is simply exposing what
> + is known in the sec. community since many years.
>
> To say that an AV scanner is a "complete failure" because it
> fails to detect a variant you just created is inane.  Each
> of the top 10 AV scanners detects well over 95% of all known
> viruses.  The AV scanners aren't perfect, but they
> definitely make a BIG BIG difference wrt malware risk
> mitigation.
>
>
ftp://agn-www.informatik.uni-hamburg.de/pub/texts/tests/pc-av/2003-04/0xecsum.txt
>
> Thierry Zoller said:
> + The solution was to make the engines a bit "smarter", i.e
> + analyse the header to determine the type and then ONLY
> + apply the signatures/heuristics which apply to the type of
> + the file (i am not speaking about the extension of the
> + file here) thus speeding up the process. Changing the
> + header just makes the smart engines look...well...  a bit
> + dumb in my regards.
>
> There are two types of people in the world:  those who
> complain about problems, and those who find solutions to
> problems.  Where's your superior AV scanner?
>
> --
> x @ bos
>
>
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